Glass sheets are quenched to provide tempering or heat strengthening in order to increase the mechanical strength of the glass and hence provide an increased resistance to breakage as compared to annealed glass. The sudden cooling process gives the glass sheets high compressive forces at their surfaces. Tempered glass sheets are less susceptible to breakage and break into small pieces that are dull and relatively harmless instead of into large pieces as in the case of untempered glass.
In tempering, quenching gas is impinged with the opposite surfaces of the glass sheet to provide rapid cooling thereof such that the finally cooled glass sheet has compressive forces at its surfaces and tensile forces at its center.
With heat strengthening, quenching gas is also impinged with the opposite surfaces of the glass sheet but at a much lower rate and thereby provides the surfaces of the glass with compressive forces but at a much lower level than is involved with tempering. Both tempering and heat strengthening can be performed on flat glass sheets such as are conveniently used for architectural purposes and on bent glass sheets such as are conveniently used for vehicle windows.
Glass sheet quenches conventionally include opposed blastheads, each of which have elongated plenum housings or banks of nozzles that are spaced from each other and supply pressurized quenching gas to a heated glass sheet positioned between the blastheads. The plenums are spaced from each other to leave room for spent quenching gas to flow away from the glass sheet and between the plenums for flow out from between the blastheads. Normally, the quenching gas is supplied in discrete jets that are spaced along the length of each plenum housing as disclosed by U.S. Pat. No. 3,936,291. Quenching jets are normally supplied in a parallel relationship to each other to provide tempering of flat glass sheets which are positioned between the opposed blastheads of the quench, extending in a perpendicular relationship to the quenching jets.
In quenching glass sheets a greater amount of quenching gas must be supplied as the glass sheets become thinner than in the case with thicker glass sheets. The pressure and power requirements in supplying the quenching gas are hyperbolic functions of glass thickness. Consequently, as glass thickness decreases, the power, size and investment required for the air supply system for the quenching gas increases rapidly. This is especially true for batch-loaded oscillatory tempering systems with one or multiple load heating capacity.
One possible solution to this increased need for quenching gas is to reduce the length of the device providing the quench to something less than the largest glass length and follow it by a full-length cooler. The glass may be passed through the quench at a rate sufficiently slow to impart the optimal degree of tempering and thereafter into the cooler where it oscillates until the next piece of glass is passed therethrough.
This approach minimizes the length of the high-powered quenching unit and therefore the size and power requirements of the drive system of the blower as well. Further energy savings can be realized by shutting the quench unit off when a glass sheet is not present therein. However, there are many problems associated with this approach. For example, two sets of blowers and two sets of ductwork are required for each air supply system. Also, the low speed required for passing the glass sheet through the quench unit to ensure the proper degree of temper makes flatness control a problem, especially on wide sheets of glass. For example, when hot glass sheets move from the furnace into the quench unit, warpage and breakage tend to occur because the leading edge of the glass is being cooled and caused to shrink, while the trailing end is still hot and in its thermally expanded condition. This tendency is more severe, the slower the travel and the wider the glass.